1. Field of the Invention
The present invention is related to ignition systems for internal combustion engines and more specifically to amplifier circuits used in conjunction with engine speed crankshaft position sensors.
2. Description of the Prior Art
Over the past several years there has been a tendency in the automotive field to eliminate conventional mechanical breaker systems which provided timing pulses to ignition systems that, in turn, supplied properly timed spark energy to the individual spark plugs of the engine. One of the more popular replcements for the standard mechanical breaker system is the Hall effect sensor in combination with a rotating ferrous shunting element attached to the distributor shaft or the crank shaft of the engine. The Hall effect sensor is electrically connected to a solid state amplifier to produce a pulsating voltage signal to the primary of an ignition coil.
A typical Hall effect sensor and associated circuit are shown in U.S. Pat. No. 3,875,920, whereby a multi-vaned shunt wheel is mounted in a distributor for rotation with the distributor shaft. As the shunt wheel is rotated, the individual ferrous shunting vanes rotate to a close proximity of the Hall effect sensor and a permanent magnet located next to the Hall effect sensor. The close proximity of any vane causes the magnetic field at the sensor to be reduced and thereby affect its electrical output signal. As the vane rotates past the Hall effect sensor, the magnetic field at the sensor increases and causes the electrical output signal from the sensor to increase. Therefore, a cyclical signal is generated by the Hall effect sensor, that has a frequency indicative of the speed at which the shunt wheel is rotating and synchronized with the wheel position. The amplifying circuit, as described in that patent, is used to drive a Schmitt trigger circuit when the output level of the Hall effect sensor rises above a fixed predetermined level. Similarly, when the signal from the Hall effect sensor passes below another fixed predetermined level, the output from the Schmitt trigger falls back to a low level.
Typically, the output of the Hall effect sensor is a cyclical signal having high and low peak values which are irregular and vary according to vane locations and from one sensor to another. In addition, the differential voltage derived from the Hall effect sensor provides an offset for the varying peak cyclical signal and this also may vary from sensor to sensor, depending upon the strength of the magnet, Hall voltage coefficient, and the value of the biasing voltage.
Variations in the peak to peak voltage are commonplace in Hall effect sensors, since the vaned shunt wheels, such as shown in U.S. Pat. No. 3,875,920 and discussed above, are not generally manufactured as high tolerance items. Therefore, a wheel may be eccentrically mounted on the shaft or may have vanes with slight bends in them which result in variations in spacing between the individual vanes and the Hall effect sensor as the shunt wheel is rotated about the shaft.
Hall effect sensors and various other types of sensors are described in paper no. 780207 entitled "A Worldwide Overview of Automotive Engine Control Sensor Technology" by William G. Wolber, pages 1-18 appearing in a Society of Automotive Engineers, Inc. publication SP-427 entitled "Automotive Applications of Sensors". Of those shown in the Wolber paper, a variable-inductance crankshaft-position sensor is one that is receiving more and more attention due to its capability of producing a static output signal. The variable inductance sensor employs a carrier frequency oscillator which is applied to a coil wound around a ferrite ring core positioned between one pole of a permanent magnet and a multilobed disc formed of a ferromagnetic material connected to the crankshaft of the engine. The position of the multilobed disc with respect to the ferrite core affects the inductance of the coil and therefore the impedance on the output of the oscillator. Whenever a high lobe of the disc rotates to a position adjacent the ferrite core, the magnetic flux field is influenced to reduce the inductance of the coil and the impedance at the output of the oscillator. A detector circuit is employed to demodulate the output of the oscillator. In order to utilize the variable inductance position sensor with prior art circuitry, it has been necessary to provide high quality control in manufacture and installation of those sensors so that the modulation signal output therefrom is always the same to insure proper ignition timing.
However, in high production manufacturing of variable inductance sensors, such quality control has been found to be excessively expensive and therefore such sensors have not been in popular usage. Variations in magnet strength, physical displacement with respect to the ferrite cores and composition thereof as well as variations in lobe heights on the disc and possible eccentricities produced by its mounting on the crankshaft, each affect the modulation signal and the resultant timing signals output from the detector circuit from sensor to sensor and from vehicle to vehicle.